Also delete the page information cache when invalidating cache of a non-existing page

Changed the usage of WikiPage::getRevision() to Title::getLatestRevID(), so that
the cache is correctly deleted when the page does not exist and it doesn't need
to create an instance of the WikiPage class to get an already-available value.

Change-Id: Ied4dedd051759503046d5461c640de3490d29c91
Merge "Also delete the page information cache when invalidating cache of a non-existing page"